The present invention relates to a means for obtaining improved braking distribution in vehicle braking systems, whether electronically controlled or mechanical.
The curve governing the relationship between axle braking for a two-axle vehicle is well known but does not take into account other sources of braking which would cause vehicles to come virtually to rest without friction brakes being employed. Such other sources of braking are primarily frictional losses in rotating parts and, more importantly, engine braking. Many heavy vehicles have means of increasing engine braking or have some form of retarder, in the form of an additional type of endurance brake which acts on the driving wheels. In such vehicles in particular, the use of combined braking seriously unbalances the braking distribution away from the ideal in which utilization of adhesion between axles is equal. In conventional vehicles, efforts are made to obtain good distribution of foundation (friction) braking but the additional braking sources generally represent an option or an afterthought and are in no way integrated into an overall braking system for the vehicle.
In a Brake-by-Wire or Electronic Braking System, electronic controls are provided in the combined braking scheme which, of course, uses electrical signalling, in order to make braking distribution near to the ideal so as to improve safety in braking. Such a system has therefore to include some level of integration of the sources of braking. This may be the full integration of a blended braking system or the lesser integration of an interlinked system in which the foundation brake controls make suitable allowance for other sources of vehicle braking--referred to hereinafter as background braking--in order to achieve a more ideal braking distribution in actuality. It is this latter type of braking system which is the subject of the present invention.
It is recognised as impossible to measure all the sources of background braking because the sources are distributed and often quite small and, as such, sensors are not available to measure the effects of what is often losses and subject to relatively significant disturbances during vehicle movement. However, it is possible to measure some of the sources of background braking directly, such as driveline torque. It is also possible to measure the combined effect of all the sources of background braking in the deceleration which is produced on the vehicle and from a study of typical vehicles to allocate this effect between axles.